Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or drawing an image on paper, film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure from a piezoelectric element, a system of jetting out a liquid droplet by generating a bubble in the ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The ink used for such inkjet recording includes an aqueous ink, an oily ink and a solid (fusion-type) ink. Among these inks, an aqueous ink is predominating in view of production, handleability, odor, safety and the like.
The dye used in such an inkjet recording ink is required to have high solubility in a solvent, enable high-density recording, provide good color hue, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing to an image-receiving material and less bleeding, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost.
However, it is very difficult to find out a dye satisfying these requirements in a high level. In particular, a dye having good cyan color hue and excellent weather-resistant fastness is being keenly demanded.
Various dyes and pigments for inkjet recording have been already proposed and are actually used, but a dye satisfying all of the requirements is not yet found out at present. Conventionally well-known dyes and pigments having a color index (C.I.) number can hardly satisfy both color hue and fastness required of the ink for inkjet recording. As for the dye capable of improving the fastness, azo dyes derived from an aromatic amine and a heterocyclic 5-membered ring amine have been proposed in JP-A-55-161856 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, these dyes have a problem of bad color reproducibility due to undesirable color hue present in the yellow and cyan regions. JP-A-61-36362 and JP-A-2-212566 are disclosing an inkjet recording ink with an attempt to satisfy both color hue and light fastness. However, in use as a water-soluble ink, the dyes used in these patent publications are insufficient in the solubility in water. Also, when the dyes described in these patent publications are used as a water-soluble ink for inkjet recording, there arises a problem in the fastness to humidity and heat. In order to solve these problems, compounds and inks described in JP-T-11-504958 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”) have been proposed. Furthermore, an inkjet recording ink using a pyrazolylaniline azo dye for improving the color hue and light fastness is described (see, Japanese Patent Application No. 2000-80733). However, these inkjet recording inks all are insufficient in the color reproducibility and fastness of the image output.
It has been also found that when an image is recorded on an inkjet special glossy paper for photographic image quality and put on a wall in a room, the image preservability is sometimes extremely bad. The present inventors assume that this phenomenon is ascribable to some oxidative gas in air, such as ozone. This phenomenon does not occur when the air flow is shut out, for example, by putting the image in a glass-made frame.
This phenomenon is outstanding particularly in the case of inkjet special glossy paper for photographic image quality and is a large problem for current inkjet recording systems where one of important characteristics is the photographic image quality.
Also, as described above, in preparing an ink solution, when only water is used, the penetrability into a medium is bad and the image is not fixed in many cases. In addition, the liquid properties necessary for hitting as an ink are often not satisfied. For solving these problems, a technique of using a water-miscible organic solvent as an auxiliary solvent is commonly employed in this field. However, mere use of a water-miscible organic solvent has been found to cause a problem that the image formed is readily blurred under high-humidity condition or the ejection stability is impaired due to drying of the ink at the inkjet head.
Accordingly, an object of the present invention is to provide an inkjet recording ink which is an aqueous ink in view of handleability, odor, safety and the like and which can ensure high ejection stability, good color hue of the obtained image, no drying of the ink at the inkjet head, excellent fastness to light and water, image quality free from defective such as fineline blurring under high-humidity condition, and good preservability of the image under severe conditions. Another object of the present invention is to provide an ink set capable of ensuring high storability and high ejection stability even after aging of the ink for a long time or under severe conditions.